Fuel cell technology shows great promise as an alternative energy source for numerous applications. Fuel cells have been investigated for use in mobile applications, such as portable computers, mobile communications, and GPS tracking devices. Several types of fuel cells have been developed, including polymer electrolyte membrane fuel cells, direct methanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. For a comparison of several fuel cell technologies, see Los Alamos National Laboratory monograph LA-UR-99-3231 entitled Fuel Cells: Green Power by Sharon Thomas and Marcia Zalbowitz.
Important challenges faced in the development of fuel cell technology include enabling quick start-up of fuel cells in cool environments (less than 10° C.) and ensuring the stable and continuous operation of fuel cells in low-humidity environments. In cool environments, the electrochemical cell stack of a fuel cell may take up to 10 minutes to reach its operational temperature. In typical consumer applications, this warm-up period is an inconvenience. However, in more aggressive applications, such as use in military environments, such a delay may affect the operation of vital communications, navigation, or global positioning equipment, presenting an unacceptable risk to the safety of military personnel and thus deterring the implementation of fuel cells in these applications.
Traditional direct liquid fuel cells vent a large portion of the heat and water they produce into the surrounding environment. Consequently, use in cool environments may be less than optimal. By venting heat from the fuel cell system, the overall temperature of the fuel cell stack is decreased, reducing its reaction kinetics and, as a result, reducing the amount of power generated by the fuel cell. In order to overcome issues encountered in cool environments, such as less than optimal power generation resulting from a reduced operating temperature, previous systems have increased the amount of platinum catalyst used in the stack and/or increased the number of cells in the stack. However, such measures may substantially increase the cost and weight of the fuel cell stack.
In dry environments, continuous operation of a traditional direct liquid fuel cell may lead to internal dehydration issues, since the exhaust stream may contain large amounts of water which cannot be fully recovered by the fuel cell's condenser before the exhaust stream is vented into the surrounding environment. If the amount of water which is vented from the system is greater than or equal to the rate at which it is produced internally, the fuel cell may experience reduced performance or failure due to dehydration. To overcome water loss issues which may arise when fuel cells are operated in high-temperature/low-humidity environments, such as in the desert, previous direct liquid fuel cell systems have been designed to operate on a water-diluted fuel (approximately 30% water by volume), rather than on 100% fuel. However, the use of diluted fuels increases the overall weight and decreases the portability of the fuel cell system, since, as compared to 100% fuel, a larger volume of diluted fuel is required to achieve the same performance and operational run-time.
Another challenge in direct liquid fuel cell technology is developing fuel cells for use in enclosed spaces or in spaces with poor ventilation. Because a fuel cell may experience fuel cross-over and incomplete fuel utilization, fuel vapor and incomplete oxidation emissions may be vented into the area surrounding the fuel cell, threatening the health and safety of persons nearby.